Quantitative Study of Degeneration of Mixed Prairie

Abstract

1. An extensive area of Mixed Prairie in the loess hills of central Nebraska was studied to ascertain the degree of degeneration that had resulted from long periods (60-70 years) of grazing. 2. The topography is mostly that of steeply rolling and hilly land. The soils, derived from loess, have A horizons varying from 6 to 20 inches in depth, carbonate accumulations in the thick B horizons, and C horizons of deep, mellow loess. Water from the 23.5 inches of precipitation penetrates the soil to 5 feet or more. 3. Ungrazed hilltops and upper slopes were covered mostly with short grasses. Ravines and ravine banks supported a post-climax vegetation of tall and mid grasses derived from True Prairie. Typical climax Mixed Prairie of taller grasses with an understory of short grasses occupied the long mid slopes. 4. Each-type of vegetation demarked different environments or pasture sites. Various degrees of deterioration of the vegetation in each site were revealed in four range-condition classes-excellent, good, fair, and poor. 5. In lowland the chief grasses which decreased under grazing were Andropogon gerardi and other tall grasses. In excellent range A. gerardi composed 58% of the perennial grasses, but decreased to 29%, and then to 3%, and finally in poor pasture to 1%. Conversely, short grasses (Bouteloua gracilis and Buchloe dactyloides) increased from 10 to 82% from excellent to fair pasture, and then decreased to 53% in poor range. Poa pratensis and Bouteloua curtipendula were other increasers. 6. Basal area of grasses increased with lowland range deterioration from 15% in excellent pasture to 51% in fair range, but it was only 21% in poor range. Organic mulch steadily decreased, as vegetation degenerated, from 8540 pounds per acre in excellent range to 634 pounds in poor pasture, and yield of perennial grasses from 2.44 tons per acre to only 0.27 ton. 7. On hillside sites Andropogon gerardi, Bouteloua curtipendula, and Poa pratensis all decreased from their respective 8, 10, and 3% composition of the vegetation to less than 1% in fair range. Bouteloua gracilis maintained a high percentage composition (50-75) throughout. Buchloe dactyloides increased from 5 to 45% in fair pasture and then decreased. 8. Basal area on hillsides increased from excellent pasture (27%) to fair range (43%) and then decreased to 18%. In the same sequence, yields of perennial grasses were 1.44, 1.33, 0.92, and 0.41 tons per acre. A yield of 5900 pounds per acre of mulch in excellent range decreased to 281 pounds in poor pasture. 9. On hilltop sites there were no tall and few mid grasses to decrease. The two short grasses composed 95% of the basal area in excellent pastures and 92% in poor ones. Here degeneration consisted largely of the fragmentation of the more or less continuous cover of excellent range to one with much bare interspace. 10. Basal area of grasses increased under grazing from 36 to 49% and then, in poor range, decreased to 20%. Mulch was reduced from 3980 pounds per acre to only 281 pounds in poor range. Yields of grasses were moderate in excellent and good range (about 1.28 tons per acre) but low (0.65 and 0.41 ton) in fair and poor range. 11. The same species of forbs often grew in different sites, but some were more or less confined to a single site. Each pasture type had a characteristic group of forbs which consistently decreased with degeneration of prairie, as Amorpha canescens, Lygodesmia juncea, and Ratibida columnaris. 12. Chief forbs that increased with deterioration of grassland were Vernonia baldwinii, Aster multiflorus, Ambrosia psilostachya, and Cirsium undulatum. Among forbs that invaded degenerated grassland were Verbena stricta, Grindelia squarrosa, and, in poor pastures, Helianthus annuus and many other weedy annuals. 13. Of 148 ranges examined, only 5% were in excellent condition, 18% were in good condition, fair ranges composed 49%, and distinctly poor ranges were 28% of the total.